Vessels must be checked in different areas of technology with respect to their operability, that is, with respect to tightness. Thus, in the chemical industry or in the process industry for example, it is important that the tightness of vessels be checked. Furthermore, it is however also necessary to check the tightness, especially of tank systems, in the motor vehicle industry.
For example, the California Environmental Authority (CARB) as well as the Environmental Protection Agency (EPA) require a check of the operability of tank-venting systems in motor vehicles utilizing on-board diagnostic means (On-board Diagnosis, OBD II). Starting with the model year 1996, leaks of the size of 1 mm or more must be detected and starting with the model year 2000, the detection of leaks starting with a size of 0.5 mm is required. Such leaks must, for example, be indicated in the motor vehicle and be stored in a memory.
U.S. Pat. No. 5,349,935 discloses a method and an arrangement for checking the operability of a tank-venting system wherein an overpressure is introduced into the tank-venting system by means of a secondary air pump. The presence of a leak is subsequently concluded from an evaluation of the course of the pressure.
It is disadvantageous with respect to this method and this arrangement that a secondary air pump is not present in all vehicles. Furthermore, a pressure sensor is necessary to evaluate the course of the pressure. The sensor is not only an additional element of the tank-venting system which can malfunction but also makes the system more expensive.
Furthermore, an arrangement for checking the operability of a tank-venting system is known wherein the overpressure is introduced into the tank-venting system by means of a pump or pressure source. The volume flow introduced is measured at a diaphragm by means of a difference pressure measurement and, thereafter, a decision is made as to whether a leak is present or not from a comparison with a programmable threshold.
It is a disadvantage of this arrangement that a measurement of an absolute volume flow is required which is compared to a threshold. This absolute measurement of the volume flow is problematic for the reason that the total of tolerances of the flow machine go into the measurement, for example, the tolerance of the output volume flow. In addition, a pressure sensor for measuring the pressure is required even in this arrangement. This sensor not only makes the entire method and arrangement more complex but also more expensive.
A further method is known for checking the operability of a tank-venting system wherein a reference leak is switched into the tank-venting system and wherein a statement as to the presence of a leak is made from a comparison of the measurements with and without the reference leak.
Also, U.S. Pat. No. 5,347,971 discloses a method for checking the operability of a tank-venting system wherein a conclusion as to the tightness of the system is reached from a comparison of the measurements with and without a reference leak.
In the two last-mentioned methods, it is problematical that a proper reference measurement is not possible between a reference leak and a leak present in the tank-venting system because a leak, which is possibly present in the tank-venting system, always affects the measurement, even when the reference leak is switched in. The reference leak is accordingly not a proper reference leak and, instead, should rather be characterized as an "offset" leak. With this offset leak, measurement inaccuracies can be expected in the check of the operability of a tank-venting system.
A method wherein the check of the operability of a tank-venting system takes place with the aid of a proper reference leak is disclosed in U.S. Pat. No. 5,390,645. In this method, the volume flow of a blower motor is split so that it simultaneously flows through a reference leak and into the tank-venting system. A conclusion as to the presence of the leak is reached from a comparison of the flow in the two flow paths wherein the flows are detected by through-flow sensors mounted in the respective paths.
However, a disadvantage of this method is that two relatively complex through-flow sensors are required to carry out the method. Furthermore, it is a disadvantage in this method that the overpressure source is mounted in the flow path of the regeneration air of an adsorption filter because this regeneration air often contains dirt and water mist, salt water and the like which can disadvantageously affect the function of the overpressure source. This operates unfavorably on the service life of the overpressure source.